Does Parylene Get Everywhere?

Following thorough research, Parylene emerged as the optimal conformal film for your application. Its consistent protective and insulating properties are particularly noteworthy, catering to a wide array of applications, from printed-circuit boards (PCBs) to medical implants to military-grade uses. In addition to these benefits, Parylene offers:

• Adherence to an exceptional quantity of substrate geometries/materials
• Biological/chemical inertness
• Bubble- and pinhole-free conformability/flexibility at film thicknesses greater than 0.5 microns
• Excellent dielectric/moistue barrier properties
• High optical clarity,
• Penetration of extremely small crevices/spaces
• Tin whisker mitigation
• Withstands autoclave-level heat

Applied through a chemical vapor deposition (CVD) process, gaseous Parylene can be deployed and adhere on any surface that touches air. Thus, it has the capacity to coat under components, inside minute substrate fissures, and inside semi-sealed areas. Unlike liquid film materials, the micron-level thinness of Parylene films generate coatings without forming bridges in tight areas.

These properties have been verified repeatedly through Parylene’s use and have been extended as application technologies improve. For your coating application, it is crucial to ensure that the Parylene film covers and adheres to all areas specified in the coating requirements. Having the most dependable evidence of the coating’s complete conformity is essential.

Verifying Parylene Conformality

Ensuring uniform Parylene coating coverage across the specified surface is crucial and may necessitate specialized inspection techniques. Electron microscopy (EM) surpasses light microscopes in resolution power, offering significantly higher magnification for detailed views of smaller objects such as Parylene-coated microelectromechanical systems (MEMS) and nanotechnology applications. By using a beam of electrons, which are stable, negatively-charged subatomic particles present in all atoms and the primary carriers of electricity in solids, EM generates high-resolution images to verify Parylene conformality.

To enhance precision in imaging, when the Parylene film thickness surpasses 200 nm, physically cleaving the coated specimen may become necessary. Cross-sectional scanning electron microscopy (SEM) can then be employed to capture images suitable for evaluating conformality. These SEM images offer insight into the quality of the Parylene coating, revealing whether it is uniform, pinhole-free, or if any gaps are present. Sequential cross-sectioning helps determine conformality (or its lack) along the surface of a single specimen or through an entire sample.

Physical cross sectioning may not work for all substrate topographies. Use of ion/electron beam ablation (I/EBA) can successfully image the Parylene film/substrate interface, to determine if the Parylene has adhered everywhere identified by the specifications.

Verifying Parylene conformality becomes proportionally more difficult as coating layers decrease in size, as with MEMS/nanotechnology applications, where layers frequently are less than 100 nm. Analysis of SEM cross-sectioning becomes more difficult under these circumstances, suffering from Z-contrast/charging effect inconsistency. These conditions can be rectified using a focus ion beam (FIB) system in conjunction with transmission electron microscopy (TEM). TEM is used to view thin specimens – like tissue sections, molecules, in addition to conformal layers — through which electrons can pass generating a projection image.

Electron microprobe analysis (EMPA) can enrich TEM imagery. Working similarly to an SEM, EMPA is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. While this technique is adept at verifying the conformality of thinner, more complex layers of Parylene, its accuracy can be challenged by ion damage, as film thicknesses diminish (>20 nm).
In such cases, using SEM images prior to- and following CVD can generate a reasonable view of coating covering and conformality. This technique is valuable in cases characterized by property changes to the substrate surface initiated by CVD. Such applications – where preservation of the precursor functionality down the depth of feature is necessary – benefit from combined (before/after) SEM imaging. Comparing prior-with-final assembly properties verify applied Parylene conformality in these cases.

A Simple Solution

These techniques are invaluable for pinpointing the precise positioning of Parylene across various layers of the coated object. In many cases, a visual inspection of the unmasked areas suffices. Observing the demarcation lines provides assurance that the coating extends over the entire board; it is an intrinsic presence that cannot be overlooked.